1. Statement of the Technical Field
The invention concerns the transportation of digital information on optical media. More particularly, the invention concerns the transportation of a plurality of eight (8) megabit per second signals in accordance with a Synchronous Optical Network (SONET) standard.
2. Description of the Related Art
A Synchronous Optical Network (SONET) provides a system for transmitting digital payload information (for example, voice, video or data traffic) over an optical fiber and/or a copper wire. The basic unit of transmission for SONET is Synchronous Transport Signal one (STS-1). The STS-1 bit transfer rate equals 51.840 megabits per second. In this regard, STS-1 provides a framing format used for the transmission of control and digital information within a SONET based network. A STS-1 frame consists of ninety (90) columns by nine (9) rows of bytes. The STS-1 frame consists of a transport overhead (TOH) and a synchronous payload envelope (SPE). The TOH occupies three (3) columns by nine (9) rows of bytes. The TOH includes section and line control information. The transport overhead bandwidth is approximately 2 megabits per second. The SPE occupies eighty-seven (87) columns by nine (9) rows of bytes. The SPE includes path overhead (POH) and digital payload information (for example, voice, video or data traffic). The SPE bandwidth is approximately fifty (50) megabits per second.
Virtual tributaries (VTs) are subsets of STS-1. VTs transport digital information at a lower bit rate than an STS-1 format. There are four (4) standard defined VT sizes suitable for a Time Division Multiplexing (TDM) Network hierarchy. These include a VT1.5 (i.e., bit transfer rate equals 1.728 megabits per second), a VT2 (i.e., bit transfer rate equals 2.304 megabits per second), a VT3 (i.e., bit transfer rate equals 3.456 megabits per second), and a VT6 (i.e., bit transfer rate equals 6.912 megabits per second). It should be understood that two or more VTs can be grouped together using a virtual concatenation technique to from a single STS entity having a higher bit rate than its constituent VTs.
Various private and governmental clients have specific bit rates at which they prefer to transmit digital payload information (for example, voice, video or data traffic) through a network, such as a TDM Network. For example, a client prefers eight (8) megabits of digital information to be transmitted through a TDM Network every second. In such a scenario, the eight (8) megabits per second signal needs to be mapped to a SONET frame. One technique that can be used for this purpose requires mapping the eight (8) megabits per second signal into a fifty (50) megabits per second SPE. However, this technique suffers from certain drawbacks. Specifically, this technique is inefficient because less than one sixth (⅙) of the SPE bandwidth is used.
Another technique that can be used for this purpose requires the mapping of the eight (8) megabits per second signal to a single VT. However, there are no appropriately sized VTs that can accommodate an eight (8) megabits per second transfer rate directly.
Yet another technique that can be used for this purpose requires employing virtual concatenation. In this regard, virtual concatenation of four (4) VT2s can form an eight (8) megabits per second signals. However, this technique also suffers from drawbacks. Specifically, this technique requires special hardware, such as an inverse multiplexer, to break the eight (8) megabits per second into four (4) VT2s and to reassemble the eight (8) megabits per second signal.
In view of the forgoing, there remains a need for a method of mapping an eight (8) megabits per second signal into a SONET frame without wasting bandwidth.